The present invention relates to the improved system for an optical recording/reproducing system, in particular, it relates to such a system which records information with extremely high density. The present invention is applied to a heat mode optical recording system, in particular, an erasable optical recording system in which a heat meltable plastic, like styrene is used as a recording medium.
In a heat meltable plastic medium, data is stored by producing a pit on a medium by melting the same by illuminating the surface of the plastic with a laser beam. That plastic medium has the advantage that an erasable recording is possible like a conventional magnetic recording system.
Plastics having main component styrene melt in high temperature. When that meltable plastic is used as a memory medium, data is stored in that medium by illuminating the surface of the plastic with a thin laser beam to provide a pit on the surface of the same. Thus, the presence of a pit shows that the data "1" is stored, and the non-existence of a pit shows that the data "0" is stored. That data stored in the plastic is read out by illuminating the surface of the same with a weak laser beam, which reflects irregularly or regularly according to the presence or non-presence of a pit, and then, the beam reflected at a pit can indicate whether the data stored in the plastic is "1" or "0".
The data thus stored is erased by melting the surface selectively by illuminating the same with a thick and strong laser beam, and cancelling a pit. When a pit is cancelled, new data can be stored again at that portion where a pit existed. Therefore, meltable plastic is an erasable optical memory medium, which can store data repetitively, like a conventional magnetic recording system.
Our previous U.S. patent application Ser. No. 362,983 filed on Mar. 29, 1982 handles that kind of optical recording system.
A conventional heat meltable optical recording system is described in accordance with FIGS. 1A and 1B, and FIGS. 2A through 2C, for the easy understanding of the present invention.
FIG. 1A and FIG. 1B show the structure of an erasable optical recording medium, which can be implemented for instance by a plastic having the main component of styrene. In FIG. 1A, the reference numeral 20 shows a substrate which is for instance acrylic resin or glass, 22 is a thin film of a plastic recording medium which is composed of a plastic in which the main component is styrene. Preferably, that recording medium is styreneoligomer which includes some dye (for instance "Oleosol first blue EL"), for absorbing optical energy sufficiently.
When the surface of that recording medium is heated selectively with a laser beam, the heated portion is melted and a pit is produced. A pit is shaped as shown by the reference numeral 26 having a ring-shaped circumferential bank 26a, and a circular flat portion 26b surrounded by the ring 26a (see FIG. 1B), because of the surface tension effect of the melted liquid and the flow of said liquid outside of a pit. After a pit is cooled, that pit shows that the data "1" is stored. That concaved pit corresponds to the data "1", and the non-existence of a pit corresponds to a data "0".
In order to facilitate the reproduction of a pit, or an erasable recording, a thin plastic film 26b must exist at the bottom of a pit, that is to say, the substrate 20 must not be exposed in air at the portion of a pit. In order to assure that the substrate 20 is covered with a plastic film at the portion of a pit, the thickness (t) of the recording medium 22 must be greater than the diameter (d) of a pit, and preferably, that thickness (t) is greater than 3 .mu.m. It should be appreciated that the heat conductivity of a styrene polymerization product with low degree is small, and therefore, a pit can be produced with a relatively low energy density even when the thickness (t) of the medium 22 is large.
If the thickness (t) is smaller than the above value, the affinity between the substrate and the recording medium must be greater than the surface tension of the melted recording medium, in order to assure that the substrate 20 is not exposed to air at the bottom of a pit. The preferable combination of the substrate and the recording medium for satisfying the above condition is that the recording medium is styrene-oligomer, and the substrate is acrylic resin or glass. If the above condition is satisfied, the bottom of a pit does not expose the substrate to air even when the recording medium is melted till the bottom of the pit, since the substrate is wet by the melted recording medium, and when the ring 26a is melted afterwards, the pit is cancelled for the next recording. If the above condition is not satisfied, the melted recording medium is shaped like a ball by the surface tension effect, and a pit can not be cancelled, thus, an erasable recording would be impossible.
The data stored in a pit is read out by illuminating the surface of the recording medium with a weak laser beam, and detecting the reflected beam from the surface. When no pit exists, no irregular reflection occurs, and therefore, the incident angle of an input beam is equal to that of an output beam, then, a photo-detector positioned in the incident angle of the output beam can only detect a beam. On the other hand, when a pit exists, an irregular reflection occurs, and the photo-detector does not detect a beam. Thus, a photo-detector can provide output data.
The recording, reading and erasing of data is described more in detail in accordance with FIGS. 2A through 2C.
The recording is accomplished by illuminating the surface of the recording medium with a laser beam having the diameter of 1-2 .mu.m (for instance, the power of that laser beam is 5 mW), and produces pits 26A, 26B, 26C, and 26D (see FIG. 2A).
The erasure or cancellation of data is accomplished by illuminating a pit and its peripheral portion with a laser beam or an LED beam (light emission diode beam). It is supposed that the diameter of a beam for erasing data is three times as large as that of a pit. When a plastic is heated by that beam, the recording medium is melted, and a pit is cancelled and is filled with melted plastic. The reference numeral 26B in FIG. 2B shows an erased pit.
When the erased portion is illuminated again by a laser beam with the diameter of 1-2 .mu.m, a pit is generated again and new data is stored. The reference numeral 26E in FIG. 2C shows a new pit.
Since the cancellation and/or the regeneration of a pit are repeatable, an erasable optical recording is accomplished by using a plastic, as a conventional magnetic recording system does. Further, an optical recording has the advantages of low noise.
However, a prior optical recording system as described in accordance with FIGS. 1A, 1B, and FIGS. 2A through 2C has the disadvantage that the recording density is not enough, that is to say, the quantity of data stored in the predetermined area of a recording medium is not relatively sufficient. That low recording density of the prior art comes from the fact that a pit is circular occupying some area (see FIG. 1B), and that a laser beam for providing a pit is not a complete spot, but has some power distribution, that is to say, when a laser beam illuminates a pit, the peripheral portion of that pit is also illuminated, and therefore, that peripheral portion can not be used for storing another pit in order to keep the necessary resolution.